1. Field of the Invention
The invention relates to a method of reducing scaling of heat transfer surfaces in an evaporation plant of a sulphate cellulose mill when sulphate cellulose is manufactured by a process in which ash containing sodium sulphate and produced in a combustion process of a soda recovery boiler is added to black liquor to be burned, before the last evaporation stage of the black liquor.
2. The Prior Art
In a sulphate cellulose process, like in all chemical processes, losses of various chemicals occur either in gaseous or liquid form. In the sulphate cellulose process, gaseous losses occur for instance in cooking plant and evaporation plant with exhaust steam, i.e. secondary steams, and in soda recovery boiler with flue gases. Liquid losses occur in washing plant, for example, from where chemicals flow out with wash water. For the maintenance of the operation of the process, the losses of chemicals shall be replaced by feeding new chemicals to the process at a suitable stage. It is especially important to keep the sulphur/sodium balance of the process at a suitable level. This is considerably impeded by the fact that chemical losses are rather different in different factories, and accordingly, it is not possible to define generally applicably an unequivocal addition of chemicals.
Sodium and sulphur losses in the chemical recovery loop are generally compensated for by adding sodium sulphate obtained from ash hoppers and an electrostatic filter of the soda recovery boiler to strong black liquor before it is burned. It is then possible to add also extra bought sodium sulphate, if the use of ash is not enough for the maintenance of equilibrium. A separate so-called make-up chemical is needed less in the present technique when closed chemical recovery loops are introduced more than before, in which various losses of chemicals are recovered and recycled in the process.
U.S. Pat. No. 4,909,899 discloses a solution in which ash is added to liquor in the chemical recovery loop after the last evaporation stage just before the liquor is fed into a soda recovery boiler. On the other hand, U.S. Pat. No. 5,112,441 discloses how ash and make-up chemicals are fed either directly into a combustion chamber of a soda recovery boiler or into a separate mixing tank, which is before the last additional concentration stage. This publication concerns a solution in which black liquor is burned at a high dry solids content of about 80% or even more. One object of the prior art technique has been that ash is added to black liquor before the last evaporation stage at a dry solids content of about 65%, whereby so-called mother crystals are produced in the black liquor before its final concentration. The mother crystals then act as crystal nuclei in the black liquor, which leads to that crystals produced at the evaporation of black liquor stick to the mother crystals and form bigger and bigger crystals. These big crystals again pass forward with the black liquor and do not stick to the wall of an evaporator unit, due to which the last evaporator unit does not scale so easily as it would do without mother crystals.
The article "Crystallizing sodium salts from black liquor" (Hedrick, Kent, Tappi Journal, December 1992) describes crystallization of sodium sulphate and sodium carbonate contained in black liquor, whereby they together form Burkeite under certain circumstances and crystallize together with the Burkeite. The article describes the behaviour of these salts in black liquor and their crystallization and other properties from the point of view of evaporation plant. The article discusses primarily the theory of crystallization and experiments made on the basis of that theory as well as an application of the solution in connection with crystallizers.
A general drawback of the prior art technique is that the salts produced in the process tend to crystallize at different stages and to scale especially heat transfer surfaces of an evaporator. There have been problems also with the operation of the mixing tank. In circumstances when a dissolution of salts is difficult also otherwise, salts introduced as an addition are not always capable of dissolving in a desired manner, and therefore, the scaling problem is maintained. Due to this, a utilization of mother crystals is also difficult, because solid crystals do not stick to mother crystals any longer, but the salts remain as small separate crystals.